Waveguide directional couplers are widely used in applications such as the combining of power from a plurality of low power signal sources to generate higher power signals, for signal sampling, for comparing the power applied to and reflected from an antenna, in noncontact phase shifting switching schemes, and the like.
In general, a waveguide branch directional coupler includes two generally parallel through waveguides which are connected together by two or more branch waveguides, as described in U.S. Pat. No. 2,558,385 issued June 26, 1951, to Purcell. The relative amount of power coupled from one of the through waveguides to the other is determined by the amount of power flowing in the branch waveguides, which, in turn, depends upon the relative dimensions of the branch waveguide and the through waveguide to which the signal is applied. The directional properties arise from spacings of the branch waveguides at multiples of one-quarter wavelength, which results in in-phase addition at a given port for one direction of propagation and out-of-phase addition (cancellation) at the other port. The couplers are often manufactured by machining slots into a solid block of metal to form a conductive housing. A separate cover closes the housing. In the past, the design and fabrication of waveguide branch couplers has been difficult because the cross-sectional dimensions of the branch waveguides differed from one coupling value to another, and in addition the center-to-center spacing between the branch waveguides remained at one-quarter wavelength (.lambda./4) at the center of the operating frequency range. Consequently, a change in coupling value which necessitates a change in the cross-sectional dimensions of the branch waveguide may also affect the center-to-center spacing between adjacent branch waveguides. Further, the change in center-to-center spacing affects the overall dimensions of the directional coupler, so that the overall dimensions of the required housing might differ from coupler to coupler. Since the exact value of coupling depends upon slight junction effects which are not readily calculable, a branch waveguide directional coupler calculated to give a desired value might, when actually manufactured, deviate slightly from the desired coupling value. In order to obtain a directional coupler having the desired coupling value, it would then be necessary to fabricate a new coupler housing having different branch waveguide cross-sectional dimensions, center-to-center spacing and overall dimensions. This procedure is time consuming and expensive.
It is desirable to have a branch directional coupler which can be inexpensively made for various different coupling values.